Protective relaying generally involves the performance of one or more of the following functions in connection with a protected power or energy system: (a) monitoring the system to ascertain whether it is in a normal or abnormal state; (b) metering, which involves measuring certain electrical quantities; (c) protection, which typically involves tripping a circuit breaker in response to the detection of a short-circuit condition; and (d) alarming, which provides a warning of some impending problem. Fault location is associated with the protection function. It involves measuring critical system parameters and, when a fault occurs, making an estimate of the fault location so that the faulted line can be returned to service as quickly as possible.
The phasor diagrams in FIGS. 1A-1E illustrate the effect of faults on the system voltages and currents. The diagrams are for effectively grounded systems, wherein the neutral is solidly grounded, and for the ideal case of a zero fault resistance (R.sub.F =0). However, they are illustrative of the effects of faults on other types of systems, e.g., ungrounded and impedance grounded systems. In the diagrams, the dotted, uncollapsed voltage triangle exists in the source (the generator) and the maximum collapse is at the fault location. The voltages between the source and fault will vary between these extremes. The diagrams depict the effects of various types of faults on the currents and voltages (represented by phasors) in the system. FIG. 1A depicts the phasors for normal, balanced conditions; FIG. 1B depicts the phasors for a three-phase fault (V.sub.ab =V.sub.bc =V.sub.ca =0 at the fault); FIG. 1C depicts the phasors for a phase b-to-phase c fault (V.sub.bc =0 at the fault); FIG. 1D depicts the phasors for a phase b-to-phase c-to-ground fault (V.sub.bc =V.sub.bg =V.sub.cg =0 at the fault); and FIG. 1E depicts the phasors for a phase a-to-ground fault (V.sub.ag =0 at the fault).
An accurate estimate of the fault location is important to the utilities, particularly in bad weather and/or rough terrain, to avoid a cumbersome search and delays in line restoration. Accuracy is particularly important for long lines because with long lines a large percentage error in the fault location estimate represents a considerable distance. Furthermore, the fault condition may be temporary, due to fault clearing and/or a change in weather conditions, and not readily visible. In successful reclosing, accurate fault location information may be necessary to locate weak spots on the line and to speed the analysis of the disturbance.
Fault location systems may be classified as two-terminal data systems or one-terminal data systems. With two-terminal data systems, voltages and currents are measured at opposite ends of the protected line(s). These systems typically are more accurate than one-terminal data systems. However, two-terminal systems have a disadvantage in that communication between the respective terminals is required. Since end-to-end communication is not always available and can be interrupted, the requirement for data from two ends of the protected line represents a disadvantage of two-terminal data systems. With one-terminal data systems, only local voltages and currents are required. End-to-end communication is not required. However, in known systems, this advantage is offset by a requirement for knowledge of the source impedance to compensate for errors introduced by the fault resistance. Since source impedance may change due to changes in network configuration, source impedance values are typically unavailable.
In one known one-terminal data system, certain initial values, both for the argument difference and the fault distance, are assumed, and the current and voltage at the fault point are determined. If these two quantities are not in phase, new values of the argument difference and the fault distance are assumed. This procedure is repeated until the calculated fault current and the fault voltage are in phase. The last calculated value of the fault distance is assumed to be the correct value. However, small changes in the assumed value of the argument difference result in great changes of the calculated fault distance. Therefore, this system in many cases provides completely incorrect values or fails to converge toward a definite fault distance.
Another known system for locating faults with respect to a single monitoring point examines the time taken for a disturbance to travel from the monitoring point to the fault and back to the monitoring point after reflection at the fault point. A problem which could arise with this system is that the reflected disturbance could be confused with other disturbances arriving at the monitoring point as a result of reflections from other points in the transmission system. This could result in the protected section of the system being unnecessarily removed from service, when the fault is outside the protected section.
U.S. Pat. No. 4,559,491, Dec. 17, 1985, "Method and Device for Locating a Fault Point On a Three-Phase Power Transmission Line," discloses a method whereby currents and voltages are measured at a measuring point arranged at one end of a section of a three-phase transmission line. FIG. 2 is a one-line schematic diagram of the disclosed system of the U.S. Pat. No. 4,559,491 and is the same as FIG. 1 of the U.S. Pat. No. 4,559,491. The transmission line section under consideration has a length DL between its end points A and B. A fault locator FL is arranged adjacent to the end point A and is connected to the line via voltage and current transformers 1, 2 that feed measuring signals u and i to the fault locator.
An embodiment of the fault locator FL is shown in a schematic block diagram form in FIG. 8 of the 4,559,491 and FIG. 8 is reproduced in the present application as FIG. 2A. The fault locator FL includes a central processing unit CPU which carries out the following functions: collection of measured values; processing of measured values; calculation of the fault distance; outputting of result of the calculation; and, a return to normal measurement conditions after the line fault has been removed. The microprocessor MP comprises a data memory DM as well as a programming memory PM. The fault locator FL shown in FIG. 2A further comprises parameter setting member PAR for setting the complex values of the line parameters (e.g., Z.sub.L, Z.sub.OL, Z.sub.A and Z.sub.B). Finally, there is a presentation unit DIS, for example, a light-emitting diode display and a printer PRI. A complete description of the mode of operation of the fault locator FL can be had by reference to column 10 beginning in line 42 and the following portions of the U.S. Pat. No. 4,559,491 which describe FIG. 9 and FIG. 10 of that patent; FIG. 9 being an explanatory diagram of the working procedure of the microprocessor in a fault locator, and FIG. 10 being in more detail in the form of a flow chart, the mode of operation of the fault locator.
Referring again to FIG. 2, the signals u and i are proportional to the voltages and currents at the point A. The line section has an impedance Z.sub.L. A fault of arbitrary type is assumed to have occurred at a point F at the distance DF from the end point A. If n=DF/DL, the line impedance between the points A and F is n.times.Z.sub.L and between the points F and B the line impedance is (1-n).times.Z.sub.L. The network located "behind" end point A has a source voltage E.sub.A and an impedance Z.sub.A. The network located "ahead of" the end point B has a source voltage E.sub.B and an impedance Z.sub.B. It is assumed that Z.sub.L is a known parameter. The patent discloses that Z.sub.A may be known or may be calculated from measured values of currents and voltages taken at the end point A before and after a fault, and that Z.sub.B may be known but, if not, should be determinable with an acceptable degree of accuracy so that its value can be set in the fault locator FL.
When a fault occurs, the fault locator estimates the unknown distance DF (or the ratio n which gives the relative distance) from measured values of currents and voltages at the end point A before and after the fault and from pre-set or calculated values of the parameters Z.sub.A, Z.sub.B, Z.sub.L. To estimate the fault location, the system determines the fault type and the measured currents and voltages are filtered for formation of their fundamental frequency components. Guided by the fault type and the complex values of the fundamental frequency components of the measured values, the impedance of the line section and the pre-set or calculated values of the impedances of the networks lying ahead of and behind the fault distance (n) are determined as the solution of the quadratic equatio n EQU n.sup.2 +B.times.n+C=0,
where n is the fault distance and B and C are dependent on the impedances and the fundamental frequency components of the measured values. (In the below description of the present invention, the fault location parameter is referred to as "m"). A shortcoming of this technique is that values of source impedance Z.sub.A and Z.sub.B are needed if the error introduced by fault resistance is to be fully compensated. (In the description of the present invention, the source impedances Z.sub.A, Z.sub.B are referred to as Z.sub.S and Z.sub.R). Source impedances change due to the changes in network configuration and information about their values is not readily available. A change in network configuration will degrade the accuracy of this technique.
U.S. Pat. No. 4,996,624, Feb. 26, 1991, "Fault Location Method for Radial Transmission and Distribution Systems," discloses a system for locating phase-to-ground faults in radial transmission and distribution lines with tapped loads, where the load currents are significant. The system measures the phase-to-ground voltage of the faulted line and the zero-sequence component of the fault current. The residual current (IR) is determined, and that current and a residual current compensation factor (k) are used to determine the positive-sequence impedance by dividing the phase-to-ground voltage by IR.times.(1+k). The distance to the fault is then determined by dividing the positive-sequence reactance by the total reactance of the faulted line and multiplying that value by the total line length. One shortcoming of this system is that it is limited to phase-to-ground faults and will not locate three-phase faults, phase-to-phase faults, and phase-to-phase-to-ground faults.
Accordingly, there is a need for an accurate one-terminal data fault location system that offers advantages over the prior art. The present invention provides such a system.